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PRINCIPAL PARTS 


OF 


AIRPLANE ENGINES 


PREPARED BY THE AIR SERVICE 


August, 1919 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1919 


1919 • i 5 

rational Advisory Committee 


for Aeronautics 
Washington, D. 0. 











Wab Department 
Document No. 939 
Office of The Adjutant General 

2 


By Exchange Navy Dept. 
Bureau Construction & Repair 
March 25 1935 




WAR DEPARTMENT, 

Washington, August h 1919 . 

The following publication, entitled “ Principal Parts of Airplane 
Engines,” is published for the information and guidance of all con¬ 
cerned. 

[062.11, A. G. o.] 

By order of the Secretary of War : 


Official : 

P. C. HARRIS, 

The Adjutant General . 


PEYTON C. MARCH, 

General , Chief of Staff. 


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PRINCIPAL PARTS OF AIRPLANE ENGINES. 


Chapter I.—The principal parts and their 
construction: 

Crank case. 

Rotary motor crankcase. 

Crank shaft. 

Cylinders. 

Cylinders of rotary motors. 

Pistons. 

Pistons for rotary motors. 

Piston rings. 

Piston or gudgeon pins. 

Connecting rods. 

Camshaft. 

Valves. 

Chapter II.—Preliminary definitions : 
Numbering the cylinders. 

Rule for determining which way en¬ 
gine will rotate. 

The firing order. 

Items to be observed before takiug 
down a strange motor. 


Chapter III.—Definitions pertaining to the 
action of the engine: 

Defined parts of the crank shaft. 

Crank shaft alignment. 

Propeller thrust to crank shaft. 

The crank path circle. 

The bore of the cylinders. 

A centered engine. 

Dead centers. 

A piston stroke. 

Directions of rotary motions. 

Stresses: 

The tension stress. 

The compression stress. 

The bending stress. 

The torsional or twisting stresa. 
Pressures: 

Normal pressure. 

Negative pressure. 

Positive pressure. 


CHAPTER I. 

THE PRINCIPAL PARTS AND THEIR CONSTRUCTION. 

CRANK CASE. 

A crank case is a cast aluminum or cast aluminum alloy case, gen¬ 
erally of the “ split type ” scraped to a semifinish inside and out, the 
division of the split or the separation of the two “halves” being 
horizontal and generally through the main crank shaft center line* 
Of the two members of the crank case, the “ upper half ” is the 
support section and is webbed to give it structural strength. It is 
sometimes referred to as the “ base section.” The “ lower half ” is 
invariably used as a catchall for oil and contains the oil sumps. It 
is frequently called the “ sump section.” 

The mating halves of a crank case in a high-grade motor are 
usually ground to fit; are not interchangeable with each other in a 
number of motors of the same style and type unless accidentally so, 
and must be handled carefully during bearing scraping and other 
motor work so that they may not be nicked or dented upon the mating 
ground surfaces. * 








6 


PRINCIPAL PARTS OF AIRPLANE ENGINES. 


The mating halves of a crank case in a high-grade motor may be 
only machined at the mating surfaces of the crank case and may 
require the application of a thin gasket to make the case joint oil- 
tight. Bear in mind that the application of a gasket changes the 
distance between the case halves, and that a gasket must not be 
applied unless the motor is designed to receive one. This also applies 
to the thickness or thinness of the gasket material used. 

Aluminum alloy is very tough and has a very high ratio of expan¬ 
sion. It, however, will not stand very much bending but will with¬ 
stand severe vibration and strains. Care is to be taken in the placing 
of any crank case upon the 44 legs ” of a test stand that the supports 
are level and in a true flat plane; otherwise when the weight of the 
motor parts is applied and the holding bolts drawn down the crank 
case will distort and misalign the entire motor. It must be looked 
upon as the motor foundation, and, like the foundation of any struc¬ 
ture, must be true and level. 

ROTARY MOTOR CRANK CASE. 

The crank cases of rotary motors differ entirely from the crank 
cases of the stationary types and are made of different material. 
This type of case is discussed under the subject of 44 Rotary Motors.” 

CRANK SHAFT. 

A high-grade alloy steel, drop forged and full machined finished 
to balance. This steel must be very tough and close grained to with¬ 
stand the vibration and inertia of the reciprocating parts. The 
usual difference should be noted between the crank shaft of the auto¬ 
mobile motor and the aviation motor. In the former the shafts are 
not fully machined or as nicely finished, with the result that there 
are unequal masses of metal which will cause vibration at high speed, 
probably going unnoticed upon the automobile with its heavy body 
and steel frame, but which if present upon the airplane would too 
violently vibrate the delicate structure of the framework. 

The special alloy steel is usually made by the use of nickel in dif¬ 
ferent proportions and with the further addition of chromium to 
obtain the desired quality. Such a steel is termed 44 nickel-chromium 
steel.” The specifications for it differ with the different makers. 

A crank shaft is an expensive unit. It is very necessary that cer¬ 
tain parts be very carefully machined, ground, and polished to re¬ 
move all scratches and to have a uniform thickness, thus to prevent 
a chance of these parts crystallizing from the large and small places 
centering the points of vibration. 


PRINCIPAL PARTS OP AIRPLANE ENGINES. 


7 


CYLINDERS. 

Aviation motors differ more widely in the cylinder construction 
than probably any other assembly unit. There may be said to be 
four common methods of cylinder making, which can be enumerated 
as follows: 

(a) The drawn-steel-tubing type .—The tube drop forged at both 
ends, one end for the bolting-down flange and the other end for the 
cylinder-head formation, exhaust and inlet valve ports being welded 
to the head. The water jacket is formed by being pressed in two 
halves from sheet steel, then applied to the machine-finished cylinder 
by welding. 

(b) The cast semisteel type. —The cylinder is cast from a steel-like 
compound, similar to the process of casting automobile cylinders, but 
with the water-jacket omitted from the side walls. The casting is 
machined to give even thickness of metal and lightest possible weight. 
The sheet-metal water-jacket is then applied and welded on. 

(c) The cast aluminum-alloy type. —This class is an aluminum 
alloy casting of the cylinders in blocks of two instead of the usual 
single unit. The double head is made separately and bolts on as the 
removable heads do in automobile practice. The cylinder bore con¬ 
tains a thin metal liner of steel, ground to exact size and pressed in. 

(d) The cast iron type. —The cylinders are cast in blocks of two of 
a very fine grade of cast iron, exactly as in the making of automobile 
cylinders, but of superior material. The water-jacket is integral 
with the cylinder block. 

CYLINDERS OF ROTARY MOTORS. 

The construction of the air-cooled cylinders of rotary motors dif¬ 
fers greatly from the above types. This class of cylinder will be 
discussed under the subject of “ Rotary Motors.” 

pistons. 

Pistons are usually a fully machine-finished aluminum-alloy cast¬ 
ing. This is the most desirable type, on account of its light weight 
and heat-conductive properties. It undergoes a large amount of ex¬ 
pansion when subjected to the operating heat of the motor, and 
special allowances of extra clearances must be made for this expan¬ 
sion. Attention should be drawn to the weight and size of the pistons 
used in an automobile motor and the aluminum alloy type of the 
aviation motor with their short piston barrels or skirts. 


§ principal parts of airplane engines. 

PISTONS FOR ROTARY MOTORS. 

This class of piston is noticeably different from the type mentioned 
above and will be described under “ Rotary Motors.” 

PISTON RINGS. 

The composition of a ring is usually a dense gray cast iron, fully 
machine finished, peened on the inner curved surface and exactly 
ground to size upon the outer curved surface. Some less expensive 
grades of rings are made by leaving the scale of the casting upon 
the inner curved surface rather than resorting to the peening for 
obtaining proper pressure against the cylinder wall. This latter 
class of ring, though common to automobile practice, may vary too 
much in cylinder wall pressure when subjected to the high rubbing 
friction and operating heat of the aviation motor, and is not recom¬ 
mended for service unless for the mere practice of assembly. When 
the ring becomes very narrow its composition must change to a 
semi-steel in order to have the proper spring qualities. Whereas, the 
usual multi-cylinder motor above the “ six ” has an oblique setting 
or inclination of the cylinder and the rings must be of the concentric 
type. The vertical cylinder mounting may permit the use of the 
eccentric ring. 

PISTON OR GUDGEON PINS. 

Pistons or gudgeon pins arc almost universally made of a special 
dense grained steel machined to size, case hardened, and finally 
grinder-finished to a polished fit. For the sake of lightness the pin 
is bored hollow. The now approved mounting of the pin is on the 
principle of fully “floating” it, i. e., having bearing in the end of 
the connecting rod as well as in the piston bosses with some endwise 
motion as well. This mounting principle is the result of the use of 
the aluminum alloy piston. As the piston expands with the operat¬ 
ing heat, the bearing “play” increases on the piston pin, loosening 
it in the bosses. On the other hand, the bronze bushing in the end 
of the connecting rod, not having the same coefficient of expansion 
as the end of the rod holding it, tends to seize the piston pin. Thus, 
with increase in operating head, the piston bosses release the pin 
and the connecting rod bushing tends to seize it. The fit of the pin 
must therefore be wonderfully accurate. On the cold motor it is j 
mild driving fit to the bosses and a bearing running fit to the bush¬ 
ing. Special instruction is required for the removal of the pin 
from a piston without injury to parts involved. 


PRINCIPAL PARTS OF AIRPLANE ENGINES. 


9 


CONNECTING RODS. 

The rods are usually made of a special alloy steel, drop forged, 
heat treated and machined to exact weight. With a dense grained 
steel and I-beam section to rod, wonderful strength is secured for 
the size used. Quite a variation in the nicety of construction exists 
between the rods of the average automobile motor and that of the 
aviation motor. In the former the rods are only machined at the 
ends, the rough forging remaining throughout the length of the rod 
proper, thus giving slightly unequal thickness of metal and making 
impossible an exact rotary balance at high speed. 

CAMSHAFT. 

The shafts are usually made of a steel known as “ low carbon ” 
but very tough. They are drop forged with the cams integral to 
the shaft, then rough ground, heat treated, tested for trueness and 
machined. The shaft is then put in a tank and copperized or copper 
plated, the parts to be hardened being ground to remove the plating. 
The case hardening comes next, and only the parts not covered with 
copper plating will receive the carbon penetration. The shaft is 
again tested for trueness and finished ground with exact care. The 
steps of this process give an idea of the value of the shaft and explain 
also the chances of error in exact cam grinding if the process is 
hurried for cheapness. 

VALVES. 

Valves are usually a ground and machine finished drop forging of 
a very dense heat-resisting steel. Some processes call for the making 
of the stem and head separately and a final welding of two together, 
then the usual machining and grinding to exact size. Valves and 
valve seats are being constructed on more of a scientific basis each 
year. With the present day small-bore high-efficiency motors, every 
small detail adds a small percentage to the final output of the motor. 
This applies greatly to the valves and much care is taken to exact 
shape and weights. If the motor is very high speed, the valves must 
be of light weight or else a very strong spring is required to hold 
them to their seats and return them to their seats, when opened, 
quickly enough to comply with the operating cycle of the motor. 
The valve stem must have wearing quality and the end be hardened. 
Long stems expand more noticeably under operating heat than short 
ones, this fact being one of the important considerations in giving 
the valve its proper stem “ clearance ” in the adjusting of valves. 

132182—19 - 2 JT 


CHAPTER II. 


PRELIMINARY DEFINITIONS. 

NUMBERING THE CYLINDERS. 

On the multi-cylinder motor, each manufacturer usually has his 
own pet method of numbering the cylinders, which fact becomes very 
confusing in this work unless regulations of a set nature are adopted. 
The three following rules, therefore, apply: 

(a) The cylinders on the stationary type motor are to be num¬ 
bered from gear end toward propeller end. If more than one 
“bank” of cylinders appear, then the bank on the left-hand (facing 
gear end of motor) bears the odd numbers and the bank on the right 
the even numbers. Thus the numbers run crosswise from left to 
right and away from gear end. 

(b) The cylinders of the Liberty will receive a special numbering, 
which will be taken up under “ Liberty Motor.” 

( c ) The cylinders of the rotary motors are numbered in regular 
order as they appear on the crankcase, but, however, opposite to the 
direction of rotation. 

RULE FOR DETERMINING WHICH WAY ENGINE WILL ROTATE. 

• 

When the motor is brought to the shop, the propeller has been 
removed and there is nothing of definite nature to indicate its proper 
direction of rotation. The following rule is to be applied in such 
cases: 

(a) Rotate the motor slowly in what is thought to be the 

proper direction. 

(b) Observe the valve operation on some one particular cylin¬ 

der. 

( c) If the period of the exhaust valve opening immediately 

precedes that of the intake, the direction of rotation is 

correct. 

THE FIRING ORDER. 

The firing order of a motor is the order in which the power strokes 
are delivered from the various cylinders to the crank shaft. In giv¬ 
ing the order of firing always start with cylinder No. 1. Place the 
10 



PRINCIPAL PARTS OF AIRPLANE ENGINES. 


11 


'cylinder numbers down one after the other as the power strokes oc¬ 
cur, thus: 1-2-3-4-7-8-5-6. A rule for determining the firing order 
may be applied as follows: 

(a) Rotate the motor in the proper direction slowly. 

(b) Observe the exhaust valves only. 

( c) The order in which the exhaust valves open is the firing 

order of the motor. 

ITEMS TO BE OBSERVED BEFORE TAKING DOWN A STRANGE MOTOR. 

No motor unfamiliar to a mechanic either in type, design, or oper¬ 
ating behavior should be disassembled or its repair started* without 
the following data, unless the motor is badly damaged physically or 
the data is to be furnished from another source. 

(a) Determine its direction of rotation. 

(b) Determine its order of firing. 

( c ) Record valve clearance adjustments each cylinder. 

( d ) Record valve timing, each cylinder. 

( e ) Record fully advanced spark timing. 

(/) Check the compression in each cylinder. 

{g) Make a notation of all missing parts. 

( h ) Note condition and- quantity of oil removed from sumps 

and any deposits the oil may contain of a metal-like 
character. 

( i ) Record any special method of arranging oiling pipes, 

wire connections of the electric system, timing of the 
starter apparatus, if one present, etc. 

( j ) Predetermine, by discussion of the matter, which assem¬ 

blies are to be removed first and have definitely in mind 
the whole order of disassembly. 

There is a psychological reason why the mechanic who carefully 
studies the disassembling of the motor will be very capable of its re¬ 
assembly. He must have it put up to him that he must take the 
•responsibility, and no one else, of the correctness of the assembly. 


CHAPTEK III. 

DEFINITIONS PERTAINING TO THE ACTION OF THE ENGINE. 


That a more accurate consideration of the action of an engine may 
be given, a few minor definitions must follow. By the use of these 
definitions, explanation is possible without chance confusion as to 
meaning. The student, therefore, should become very familiar with 
the definitions. 

DEFINED PARTS OF THE CRANK SHAFT. 

Though the crank shaft is one integral piece of metal, certain desig¬ 
nated portions bear different names. Most of these terms are 
“ trade ” terms which have been evolved from the shops. The draw¬ 
ing and explanations give more or less clearly their application. 



The above drawing gives two throws of a multiple throw crank 
shaft. Thus, a crank shaft is said to be “ one throw ” or “ two throw,” 
etc., the word “ throw ” being used like a contractor might use the 
word “ brick,” it is never pluralized. 

A journal is a highly polished portion of the crank shaft, truly 
circular in section, intended and designed to ride in a bearing. 
Since a perfectly circular cross section is nearly impossible, a jour¬ 
nal is not always as round as it is intended it should be. Wear also 
brings an “out-of-round” condition. The limit for out-of-round- 
12 

































PRINCIPAL, PARTS OF AIRPLANE ENGINES. 


13 


ness is 0.0005 inch. When this limit is reached the journal will re¬ 
quire regrinding or truing up in some fashion. 

The fillets between the journal and web provide means for securing 
a slight end thrust to the bearing. This end thrust, however, is 
not enough to counter the end thrust produced by a propeller. A 
plain journal bearing is strictly a “radial” bearing. It will take 
loads acting vertically to its centerline only. 

The throw measurement, J, is taken vertically between the center 
lines, B and C. 

CRANK SHAFT ALIGNMENT. 

When the shop mechanic speaks about the alignment of the crank¬ 
shaft, he generally speaks about the shaft 44 being in 4 line ’ ” or 44 out 
of 4 line,’ ” the word 44 alignment ” not being familiar to him. The 
alignment of a crank shaft is dependent upon three things, for—• 

(a) It may be bent, its center line not a true straight line. 

(b) To crank-pin center line skewed; not parallel to the main 

center line. 

(c) The angle between the throws may not be equal. For 

example, a two-throw shaft may be intended when in 
alignment to have its throws directly opposite, but in¬ 
stead the angle on one side may be 183 degrees and on 
the other ITT degrees. Obviously, the shaft has been 
slightly 44 wound up.” It has received a severe 44 twist v 
or 44 torque.” 

PROPELLER THRUST TO CRANK SHAFT. 

Unlike the crank shaft of the automobile motor, the aviation motor 
crank shaft receives a large end thrust. This thrust is either a 
44 pull” or a 44 push.” Whether it is one or the other, will depend 
on whether the motor is mounted as a 44 pusher ” or a 44 tractor.” A 
tractor propeller pulls itself into the air and is in front of the motor. 
A 44 pusher ” does exactly as the term implies, and is mounted in the 
rear of the motor. The same motor may be used as a tractor or 
pusher merely by turning it around and mounting a different pro¬ 
peller on it. Now, the end thrust of the crankshaft, due to the 
action of the propeller on it, must be provided with a thrust bearing. 

This bearing is usually at the propeller end of the shaft and is of 
the annular ball-bearing type. It is adjustable by the use of shim¬ 
like washers. It should be adjusted so as not to interfere with the 
shaft alignment with its bearings endwise. In other words, the shaft 


14 


PRINCIPAL PARTS OF AIRPLANE ENGINES. 

must have a certain, limited amount of end play. This end play 
allows for alternate self-adjustment between shaft and crank case 
bearings due to the effects of heat expansion acting on either one or 
the other. Say this total end play is one-sixteenth inch, and say, 
further, that the propeller is a tractor. The propeller will tend to 
wear the thrust bearing so that the shaft moves toward it, therefore 
the thrust-bearing adjustment must be such that three-sixty-fourths 
inch of the total end play is toward the propeller. 

THE CRANK-PATH CIRCLE. 

If any point upon the crank-pin center line be attached and the 
crank shaft allowed to revolve about its main center line, the point 
so selected will describe a circle. This circle, the path of the crank 
pin, is called the crank-path circle. It is a very important circle, as 
it is by reference to it that the valve timing of a motor is laid out. 

THE BORE OF THE CYLINDER. 

That portion of the cylinder interior which has been machined and 
ground, finished almost to a polish, circular in cross section, is 
called the “bore.” The bore is also defined as that portion of the 
cylinder interior forming a guide for the travel of the piston. It 
is obvious that the bore must be truly circular in form, unscratched, 
unnicked, or free of any malformation, if the piston rings are to 
travel freely and gas-tight within the cylinder. The axis or center 
line of the bore is called the cylinder center line. 

A CENTERED ENGINE. 

Most aviation motors are the “ centered engine ” type. That is, 
if the cylinder center line is produced toward the crank case, it will 
intersect the main crank shaft center line. The intersection will be 
at right angles. If it should fail to intersect the crank shaft center 
line, the engine would be called “ an off-centered engine.” 

DEAD CENTERS. 

When the cylinder center line intersects the crank-shaft center 
line, it also intersects the crank-path circle at two points. These 
points, called “ dead centers,” have distinguishing names. The one 
nearest the cylinder is called “top center,” the one farthest from 
the cylinder, “bottom center.” Reference is also made of them as 
“top dead center” and “bottom dead center.” 


PRINCIPAL PARTS OP AIRPLANE ENGINES. 


15 


A PISTON STROKE. 

As the crank pin travels about the crank pin circle, the piston 
travels up and down in the cylinder. When the crank pin moves 
from top center position to bottom center position, the corresponding 
movement of the piston is called a “ piston stroke.” A piston stroke 
is then a total movement of the piston in one given direction. One 
revolution of the crank shaft produces two piston strokes, one down 
and one up. Care must be exercised not to confuse a piston stroke 
with a cycle stroke. 

DIRECTIONS OF ROTARY MOTIONS. 

When an observer watches the rotating member of a piece of ma¬ 
chinery he generally desires to express the direction of rotation. 
This is done by stating, as viewed from the specified station point, 
that the motion is clockwise or counterclockwise, depending on 
whether or not the motion corresponds with the hands of a watch. 
With the aviation motor, the motion of the main crank shaft is de¬ 
termined from the pilot’s seat, whereupon it is said to rotate either 
clockwise or counterclockwise. On most motors the propeller is 
directly connected to the crank shaft, but on some a reduction gear 
train exists between crank shaft and propeller drive shaft. On this 
latter class the propeller motion is frequently opposed to the crank 
shaft motion. 

STRESSES. 

When loads or forces act upon the members or parts of an engine 
they subject the members or parts to a state of strain. Such a state 
of strain is called a “ stress.” There are a number of common stresses 
that the student should be familiar with and which are frequently used 
in the explanation of motor and airplane parts. The following is a 
list of the usual stresses: 

(a) The tension stress .—When an attempt is made to break a rope 
by pulling on the two ends of it, the rope is being subjected to a 
“ tension stress.” This is the stress a bolt receives when it is 
tightened, pulling two mating flanges together. 

(b) The compression stress .—If a stick is stood vertically and a 
load placed upon the top end of it, the stick will be placed under a 
« compression stress.” Such a stress tends to “ bow ” or shorten the 
length of the stick. The connecting rod of the motor is built to with¬ 
stand a great compression stress. 


PRINCIPAL PARTS OF AIRPLANE ENGINES. 























































PRINCIPAL PARTS OF AIRPLANE ENGINES. 17 

(c) The hendmg stress. —If a plank is supported at both ends and 
a load placed in the middle, the plank is acting against a “ bending 
stress.” In reality a bending stress is the result of the counteraction 
of two stresses—the compression stress and the tension stress. For 
if the plank in the above illustration has any appreciable thickness, 
then as it bends down under the load, all the fibers above its center 
line will be subjected to a compression, while all fibers below the 
center line will be subjected to a tension. 

(d) The torsional or twisting stress. —If a rod is taken in both 
hands and an attempt is made to twist the rod, wind it up, the rod is 
resisting a u torsion stress.” This is one of the main stresses the 
crank shaft receives. It is the stress*a line shaft in a shop receives. 

■m i 

PRESSURES. 

For convenience of explanation, it is often necessary to refer to the 
“pressure” in the combustion chamber or some other part of the 
motor. To this end, we explain three general pressure terms in the 
following which are frequently used: 

(a) Normal Pressure. —The pressure of the atmosphere at a sea 
level elevation upon the surface of the earth. This is equal to an 
average of 14.7 pounds per square inch (760 mm. of mercury in the 
barometer). Normal air pressure is always recorded as 0-pounds 
pressure on the steam gauge. 

(l>) Negative Pressure .—A pressure less than a normal pressure 
and extending toward a vacuum. When the handle of a suction 
pump rapidly draws the piston up, it subjects the space below the 
piston to a negative pressure. 

(<?) Positive Pressure. —A pressure greater than normal. It is 
usually a “ bursting ” pressure, such as the pressure in a steam boiler, 
automobile tire, or in the gasoline-engine cylinder during the power 
stroke. 

When an airplane ascends to a great height it encounters an air 
pressure which becomes more negative in character with relation to 
the usual air pressure of the earth’s surface. This lessening of the 
air pressure with increase of altitude has an important effect upon 
the motor. 


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